Directed self-assembly (DSA) is a technique in which copolymers such as diblock copolymers (BCP, also referred to herein as block copolymers or block polymers) containing dissimilar and non-intermixing blocks self-segregate into domains of homogeneous blocks. These domains may yield random patterns or, when directed, give well-defined and highly regular structures dictated by the molecular weight of each block. The ability of DSA to provide very small (sub-20-nm) features has quickly moved this technology into consideration as a viable option for integrated circuit production and semiconductor manufacturing processes.
DSA has also been investigated as a method for preparing nano-structured surfaces with unique surface physical properties. Possible applications include changing the hydrophobicity of surfaces due to incorporation of nano-structures and providing sites for unique chemical catalysts. DSA has promising applications in biomedical areas, including: drug delivery; protein purification, detection, and delivery; gene transfection; antibacterial or antifouling materials; and cytomimetic chemistry.
Block copolymers have been extensively investigated because of their diverse self-assembled structures such as spheres, cylinders, gyroids, and lamellae depending on the volume fraction of the blocks (F), the degree of polymerization (N), the Flory-Huggins interaction parameter (X, or chi), and the molecular architecture of the block copolymers. These nano-structures have been widely used for various applications such as templates, membranes, optical materials, and data storage media.
Compared with linear diblock copolymers, diverse architectures of copolymers such as miktoarm star copolymers or “star-shaped” copolymers have been explored because of the expectation of different morphologies or physical properties induced by different molecular architecture. Recently, it has been shown, for example, that unimolecular micelles composed of star-shaped copolymers have a tendency toward higher stability than the micelles made of diblock copolymers because the arms of the star-shaped copolymer are covalently linked to the core.
As mentioned above, the ability to self-assemble is dependent on the Flory-Huggins Interaction Parameter (X). Higher values of X (i.e., high-chi) allow for lower molecular weight polymers to assemble, leading to smaller block domains and hence feature sizes, since the natural feature pitch (Lo) of lamellae-forming diblock copolymers is proportional to the degree of polymerization. It also allows for greater thermodynamic driving force to direct assembly onto either physically or chemically differentiated surfaces. To meet the needs of applications such as magnetic storage and semiconductor devices, many recent efforts have been aimed at achieving long-range ordering, good feature registration, and accurate pattern placement with very few defects. For example, a thin film of polystyrene/poly(methyl methacrylate) diblock copolymers (PS-b-PMMA) can be spin-cast from a dilute toluene solution, then annealed, to form a hexagonal array of poly(methyl methacrylate) cylinders in a matrix of polystyrene (K. W. Guarini et al., Adv. Mater. 2002, 14, No. 18, 1290-4). Patterns of parallel lines have also been produced using PS-b-PMMA on chemically nano-patterned substrates (S. O. Kim et al., Nature, 2003, 424, 411-4).
Although there have been reports of using blends of diblock copolymers with the corresponding homopolymer(s) in forming patterns via directed self-assembly (U.S. Patent Application Publication No. 2008/0299353 A1), it is believed that there could be advantages in using block copolymers that are substantially free of homopolymer contaminants so that the composition of such blends can be more precisely controlled. However, it can be quite difficult to achieve the desired level of purity of the diblock copolymer without resorting to complex time and resource intensive procedures and/or without sacrificing yield. Examples of attempts to achieve this desired end result are disclosed in U.S. Pat. No. 7,521,094; U.S. Patent Application Publication Nos. 2008/0093743 A1; 2008/0299353 A1; 2010/0294740 A1; and PCT Publication No. WO 2011/151109. However, none of these procedures produced a product suitable for DSA applications.
Alternate architectures containing diblock segments have been examined for use in DSA, for example, linear triblock structures (U.S. Patent Application Publication No. 2013/0230705 A1), which exhibit advantages in assembly. Assembly properties of complex, asymmetric star structures have also been examined and found to be kaleidoscopic in phase diagram and morphological properties. Star architectures comprising identical diblock arms constrained by tethered chain ends to a single carbon atom gives entropic advantages in assembly, allowing for more regular assembly at smaller critical dimension versus that provided by the corresponding simple diblock systems. Star architectures also give more regularly ordered assembly by comparison with linear triblock, or simple diblock systems. It is known in the art (U.S. Patent Application Publication No. 2013/0230705 A1) that linear triblock architectures can provide advantages in assembly by comparison with corresponding diblock systems. For DSA utility, however, extreme regularity is the figure of merit, not “difficult-to-control” complexity. Thus, there is a need for a star-shaped copolymer which meets the requirements of DSA.